Results 1 to 4 of 4

Math Help - How do I make R2 the subject of this formula?

  1. #1
    Newbie
    Joined
    Feb 2011
    Posts
    11

    How do I make R2 the subject of this formula?

    Hi,

    How would I go about making R2 the subject of the formula on the following Wiki page, it's the one under Applications - Voltage Source (Vout = ...)? I don't know how to include the formula on here. Sorry !

    Link to page - Voltage divider - Wikipedia, the free encyclopedia

    Thanks,

    Becca.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    MHF Contributor

    Joined
    Apr 2005
    Posts
    15,792
    Thanks
    1532
    Are you referring to
    V_{\text{out}}= \frac{R_2}{R_1+ R_2}V_{\text{in}}?

    Multiply both sides by R_1+ R_2 to get
    V_{\text{out}}(R_1+ R_1)= V_{\text{out}}R_1+ V_{\text{out}}R_2= R_2V_{\text{in}}

    Now, get those 2 terms including [tex]R_2[tex] alone on one side by subtracting R_2V_{\text{in}} and V_{\text{out}}R_1 from both sides:
    V_{\text{out}}R_2- R_2\V_{\text{in}}= R_2(V_{\text{out}}- V_{\text{in}})= -V_{\text{out}}R_1
    Finally divide both sides by V_{\text{out}}- V_{\text{in}}

    R_2= \frac{-V_{\text{out}}}{V_{\text{out}}- V_{\text{in}}}R_1= \frac{V_{\text{out}}}{V_{\text{in}}- V_{\text{out}}}R_1.
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Newbie
    Joined
    Feb 2011
    Posts
    11
    Quote Originally Posted by HallsofIvy View Post
    Are you referring to
    V_{\text{out}}= \frac{R_2}{R_1+ R_2}V_{\text{in}}?

    Multiply both sides by R_1+ R_2 to get
    V_{\text{out}}(R_1+ R_1)= V_{\text{out}}R_1+ V_{\text{out}}R_2= R_2V_{\text{in}}

    Now, get those 2 terms including [tex]R_2[tex] alone on one side by subtracting R_2V_{\text{in}} and V_{\text{out}}R_1 from both sides:
    V_{\text{out}}R_2- R_2\V_{\text{in}}= R_2(V_{\text{out}}- V_{\text{in}})= -V_{\text{out}}R_1
    Finally divide both sides by V_{\text{out}}- V_{\text{in}}

    R_2= \frac{-V_{\text{out}}}{V_{\text{out}}- V_{\text{in}}}R_1= \frac{V_{\text{out}}}{V_{\text{in}}- V_{\text{out}}}R_1.
    Hi HallsofIvy,

    Thank you for your reply, however I was referring to the very last equation on the page. The one with "R2||RL" in it.

    I'm sorry I can't write it up like you do. I'm a noob at this

    Thanks,

    Becca.
    Follow Math Help Forum on Facebook and Google+

  4. #4
    MHF Contributor

    Joined
    Apr 2005
    Posts
    15,792
    Thanks
    1532
    Then I'm not sure what you are asking. That particular equation is defining " R_2||R_L" as meaning \left(\frac{1}{R_2}+ \frac{1}{R_L}\right)^{-1} and then noting that it happens to be equal to \frac{R_2R_L}{R_1+ R_L}. If you tried to solve \left(\frac{1}{R_2}+ \frac{1}{R_L}\right)^{-1}= \frac{R_2R_L}{R_1+ R_L} for R_2 everything would cancel out- it is an identity that is true for all R_2 and R_L.
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Make e the subject of the formula
    Posted in the Algebra Forum
    Replies: 5
    Last Post: February 5th 2011, 11:43 AM
  2. Make h the subject of the formula
    Posted in the Algebra Forum
    Replies: 1
    Last Post: June 1st 2010, 05:33 AM
  3. Replies: 2
    Last Post: September 7th 2009, 08:54 AM
  4. Make Y the subject of the formula
    Posted in the Algebra Forum
    Replies: 15
    Last Post: June 20th 2009, 09:37 PM
  5. Make x the subject of the formula (:-[)
    Posted in the Algebra Forum
    Replies: 1
    Last Post: June 24th 2008, 06:06 AM

Search Tags


/mathhelpforum @mathhelpforum